1. Field of the Invention
The present invention generally relates to the art of coating metal substrates with polymer layers and more particularly to a process and apparatus for bondingly connecting a polymer layer and a large metal substrate having a substantially plane or curved surface that is to be made resistant to corrosion.
2. Description of the Prior Art
Various methods are known to coat metals with polymer compositions or plastics so as to protect the metal surface against corrosion. As many synthetic polymers that would be desirable as anti-corrosion strata cannot be processed commercially in the form of paints or lacquers, various coating methods have been developed during the last two decades. Representative examples are plastisol coating, the fluidized bed method including electrostatic spraying, flame spraying, cast or rotational lining, vacuum coating and the like. This art is summarized for example in Europlastics Monthly (February/March, 1974, pages 59-63). A problem common to most of these prior art methods that involve heating and substantial fusion of the polymer is the appearance of tensions within the coating believed to be due to shrinking of the polymer coat when the latter solidifies.
Use of preformed polymer strata, e.g. films or sheets that are cemented or otherwise adhesively bonded onto the substrate without overall fusion or melting of the preformed polymer stratum would obviate shrinkage tension problems and substantial efforts have been made to develop suitable adhesive compositions.
One of the most promising technologies in this field is based upon the so-called heat or melt bonding adhesives. Many compositions of this type are disclosed in the literature and U.S. Pat. Nos. 2,405,950, 2,838,437, 2,953,551, 3,027,346, 3,264,272, and 3,267,083 are mentioned but as examples. Modification of the polymer chain of a normally non-bonding polymer, such as olefin, e.g. polyethylene, by carboxylic groups (free acid groups, esters or salts) introduced, for example, by copolymerizing alkylene, e.g. ethylene, and an .alpha., .beta.-unsaturated carboxylic acid, e.g. acrylic acid, is one of the commercially more important methods for producing heat or melt bonding polymers capable or serving as an intermediate bonding layer between a metal substrate and a polymer coating layer.
The application of heat or melt bonding layers for coating of metal substrates is disclosed, for example, in British patent specification No. 864,879, in French Pat. No. 2,012,338 and in U.S. Pat. No. 3,981,762.
A general approach of the prior art heat or melt bonding techniques for metal coating purposes can be summarized as follows: metal substrates are coated with an organic polymer composition by providing a polymer stratum that includes a heat or melt bonding composition in a contacting relation with the metal substrate, heating and subsequently cooling the metal substrate so as to form a strong interbonding connection between the bonding composition and the metal substrate. Various heating methods have been used in these prior art processes, including electromagnetic radiation, such as infrared radiation (3.multidot.10.sup.11 to 3.8.multidot.10.sup.14 cycles per second).
High frequency electromagnetic radiation in the range of megacycles (10.sup.6) per second to gigacycles (10.sup.9) per second is well known for many heating purposes including thermoplastics welding, bonding, curing and the like. The general concept of high frequency heating including high frequency welding of plastics seems to be based upon the concept that non-conductive organic insulating materials are heated in the high frequency field of a capacitor, e.g. by arranging the material that is to be heated between a pair of capacitor plates of a high frequency circuit. On the other hand, electromagnetic induction heating of metals is well known in the art of metallurgy and conventional metal melting devices operate in the frequency range of from about 200 to 20,000 cycles per second.
In accordance with the present invention it has been found that substantial advantages can be achieved by using electromagnetic radiation within a specified frequency range in the heating step of the prior art heat or melt bonding techniques for coating metal substrates as defined above. As will be explained in more detail below, such radiation will permeate a polymer stratum without substantial heating thereof and yet will cause a localized surface heating of a metal substrate arranged in contact with and under the polymer stratum.